Everything is backwards in Cincinatti... This sexual gesture involves the male subject to be "titty fucking" the female subject backwards. The female's view now reveals the male's rear end and right below it, his scrotum in the shape of a bow-tie, right where a bow-tie would normally rest.

This is the official definition from the director/co-writer, Judd Apatow.

You shit on someone's chest (or someone shits on yours) and you smush it all around with your ass cheeks, leaving a bow-tie shaped shit-smear

Having run out of disgusting, non-erotic things to do, I decided to give her a cincinatti bow-tie. It didn't really turn me on, and I'm pretty sure she hated it, but it did sort of look like a bow-tie... so that was neat.

Creating a bow like tie out of poop after you have pulled out of a girls ass while having anal sex with her.

The cincinatti bow tie is where you are fucking a chick in the ass and you pull out with a little poo on your doodle and then lay it on top of her ass crack and sway it back and forth so the poo spreads like a windshield wiper creating the bow tie with her balloon knot being the tie knot.

The Cincinatti bow-tie is when a woman, who is on her period, sits on the mans face so that her clitoris is in line with his mouth, thus placing the vagina in the neck area. During the "deed" she leaks blood which trickles down his neck making a "cincinnati" (RED) bow tie mark. (He didn't know she was on her period until he woke up the next morning with a Cincinatti bow-tie.)